Studies of the factors regulating myocardial perfusion have been undertaken in both the awake dog and pig. Animals with normal size hearts and with left ventricular hypertrophy are being studied. Myocardial flow distribution is measured with 7-10 micron microspheres during periods when flow is limited by a temporary stenosis. To date the studies have demonstrated a marked maldistribution of flow favoring the epicardium with an associated decrease in perfusion pressure distal to the aera of stenoses. This is most marked in the hypertrophied heart. In addition dogs have chronic collateralization produced by ameroid constrictors placed on the left circumflex and right coronary artery are being studied during exercise. In this model a perfusion defect has been demonstrated in the posterior papillary region and effects of various pharmacologic agents including nitroglycerin are to be evaluated.